Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and devices used for seismic surveys and, more particularly, to methods of firing seismic sources during such surveys.
Discussion of the Background
Due to continued interest in developing new oil and gas production, large financial, technical and human resources are used for seismic surveys capable of generating a profile (image) of underground geophysical structures. While this profile does not provide an accurate location for oil and gas, it suggests, to those trained in the field, the presence or absence of oil and/or gas.
In seismic surveys, a seismic source generates a seismic wave (an event known as a “shot”), which penetrates the underground structure and is reflected back by interfaces between layers through which the seismic wave propagates with different velocities. Seismic receivers detect the reflected waves that carry information about the underground structure (e.g., location of the reflecting interfaces).
FIG. 1 depicts schematically a land seismic exploration system 10 for transmitting and receiving seismic waves intended for seismic exploration in a land environment. At least one purpose of system 10 is to determine the absence, or presence of hydrocarbon deposits 14 or at least the probability of the absence or presence of hydrocarbon deposits 14. System 10 comprises a seismic source 11 (e.g., a vibrator) operable to generate a seismic signal (seismic waves), a plurality of seismic receivers 12 (or geophones) for receiving seismic signals and converting them into electrical signals, and seismic data acquisition system 20 for recording the electrical signals generated by seismic receivers 12. Seismic source 11 and seismic data acquisition 20 can be located in, for example, vehicles/trucks 13a and 13b, respectively. Source 11, seismic receivers 12, and data acquisition system 20, can be positioned on the surface of ground 15, and be interconnected by one or more cables 17. Source 11, seismic receivers 12, and data acquisition system 20 may also communicate wirelessly using antenna 22, 24, and 26, respectively.
A seismic wave generated by source 11 may include surface waves 34, and ground waves 36. The ground waves 36 may generate reflected waves 38 when they reach an interface 37 between two geological layers, the hydrocarbon deposits 14 or other underground reflecting structures such as 16 or 18. Each seismic detector 12 receives both surface waves 34 and reflected waves 38 and outputs an electrical signal accordingly.
Conventionally, a new shot is not fired while the seismic receivers still detect reflections from a previous shot. In order to shorten the duration of seismic data acquisition, a series of methods decreasing time necessary to fire all the shots has lately been developed and tested. For example, shots are fired simultaneously but physically far apart, or have different phases, or only waves that have different frequency ranges overlap in time (a method known as slip-sweep).
Due to the high cost of seismic surveys, there is a continuous desire to improve productivity (e.g., decrease their duration) and data quality. Accordingly, it would be desirable to provide systems and methods to speed up seismic surveys without compromising seismic data quality.